Currently used techniques for color projection displays tend to be relatively inefficient in their light utilization. Such low efficiency limits the brightness of the display, which in effect limits the acceptable amount of ambient lighting in a viewing environment.
In certain presently used designs, light from a spectrally broad source is colected by a condensing lens and illuminates a spatial light modulator system. The spatial light modulator system comprises a two-dimensional array of pixels and the amount of light transmitted through each pixel is controlled electronically. A projection lens then images the array of pixels on a viewing screen, the magnification of the displayed image being determined by the particular characteristics of the projection lens. The light impinging on each pixel of the spatial light modulator is spectrally relatively broad (e.g., white light). Therefore, unless the system is modified to distinguish colors, the display will be capable of only displaying black and white images.
In many current systems used to modify such a system so that it is capable of displaying color images, each pixel of the spatial light modulator is divided into three sub-pixels having equal areas. Each of the three sub-pixels is covered with a micro-color filter having a different spectral transmittance. For example, the filters are chosen such that one filter transmits only red light, another filter only green light, and the third filter only blue light. The transmittances of the three sub-pixels of each pixel of the spatial light modulator can be controlled independently, resulting in the ability to display a color image.
The inefficiency of the above approach can be seen by considering the following factors. The light illuminating a full pixel essentially is white light and, consequently, the light impinging each sub-pixel is also white light. The red filtered sub-pixel will transmit only red light, absorbing all of the incident green and blue light. Likewise, the other two sub-pixels will transmit only its corresponding color, absorbing the other two colors. It is apparent that this approach utilizes, at most, only one-third of the available light impinging on the modulator, and absorbs the rest.
Furthermore, state-of-the-art microcolor filters required to produce acceptable color images are only approximately 33% efficient in transmitting the color that they are designed to transmit. Therefore, the overall light utilization of current color projection displays is only about 10%.
One approach for improving the efficiency of color projection displays is found in U.S. Pat. No. 5,161,042 issued on Nov. 3, 1994 to H. Hamoda. In accordance therewith, the spectrally broad input light is supplied to three dichroic mirrors which reflect three different color components, e.g., red, green, and blue, in different directions, i.e., at different angles with respect to each other. The reflected components are then supplied to an array of lenses for focusing the different color components so as to converge light beams of similar wavelength ranges for transmission through a liquid crystal display element so as to form combined color images on a display screen. A further U.S. Pat. No. 5,264,880, issued on Nov. 23, 1993, to R. A. Sprague et al., discloses a similar approach to that of Hamoda wherein the dichroic mirrors are replaced by a blaze grating for dispersing the color components of light received thereat into a spectrum of different colors at different angles relative to each other.
It is believed that, while such approaches can be used, the losses of energy of each color component are sufficient reduce the efficiencies of such systems and to show the need for further improvement in display systems which would minimize such losses so as to provide nearly total use of the received energy across the color spectrum in the imaging display process, i.e., an optimization of the efficiency of the system.
In accordance with a preferred embodiment of the invention which achieves such improved operation, received light having a relatively broad spectrum illuminates a multi-level optical phase grating so as to disperse each of the color components contained therein into a plurality of different diffraction orders. The diffraction orders of each color component are then focussed onto a zero-order phase shift element which phase shifts only the undiffracted light (i.e., the zero diffraction order) with respect to the diffracted light (i.e., the higher level diffraction orders). The output of the zero-order phase shifter is then imaged onto a display having a plurality of pixels, each pixel having sub-pixel regions assigned to transmit different color components of light. The depths of the phase grating element and the zero-order phase shifter are suitably selected so that they are practical for manufacture and so that the area of chromaticity space for the color components at the image plane is maximized.
The use of such a combination of multi-level phase grating and a zero-order phase shifter, having suitably determined depths, provides desired color components at each pixel in which essentially little or no energy is lost, which color components are then suitably combined to provide a color image at each of the pixels of the display which is considerably brighter than that available using prior known systems.